1. Technical Field
The present invention relates to a post-processing apparatus for paper sheets and an image forming system having the post-processing apparatus.
2. Description of Related Art
A recent image forming apparatus of electrophotographic system and the like, having a high speed performance, multiple functions, and a network function, is widely utilized for printing apparatus, by being connected to a large capacity paper feeding apparatus and large capacity stacker apparatus. In cases of using the image forming apparatus as the printing apparatus, by connecting a post-processing apparatus for sorting, stapling, punching to file, and book-binding printed materials (image formed paper sheets), it is possible to execute printing through post-processing by one unit of apparatus.
As the above-mentioned post-processing apparatuses, various types of apparatuses having functions of a punching process to perforate for filing the sheets, a stapling process for binding a bundle of sheets with staples, a folding process to fold the bundle of sheets, and the like are conventionally provided.
The stapling process and the folding process are executed by conveying, with a conveyance unit, the sheets carried from the image forming apparatus onto a stacking section such as a stacker to temporarily accommodate the sheets, and stacking (accumulating) a prescribed number of sheets in the order of being conveyed.
In cases of conveying sheets to the stacker by the conveyance unit, when ejecting the sheets having low stiffness such as thin paper sheets, leading edges in ejecting direction of the sheets may hang down. In cases where the leading edge hangs down, the leading edge may be curled by contacting the sheets previously ejected onto the stacker or the accumulation surface of the stacker to cause a failure accumulation. Further, the hanged down leading edge may push out the previously accumulated sheets before completing the sheet ejection, by contacting the previously accumulated sheets.
To solve this problem, a sheet ejecting apparatus is disclosed having a control means which switches between an operation mode for imparting stiffness to the sheet to be ejected and an operation mode not for imparting stiffness, for example, in Unexamined Japanese Patent Application Publication No. 2008-308251 (Patent Document 1).
Patent Document 1, aims to improve alignment of accumulated sheets at sheet ejection and prevent damages to the sheets, by changing a condition of stiffening section between imparting stiffness and not imparting stiffness according to information of paper pressing direction.
As described above, in cases where a process, such as the stapling process, is executed by accumulating the prescribed number of sheets on the stacker, the sheets are ejected and stacked with the conveyance unit provided in front of the stacker. Since, between the conveyance unit and the accumulation position of the stacker, any member to control orientations of the sheets is not provided, the above described hanging down of the sheet edge may occur at the ejection to cause a failure stacking.
To impart stiffness on the sheets according to the sheet conditions, as described in Patent Document 1, with respect to this hanging down of the sheet edge, is effective to improve alignment of accumulated sheets at sheet ejection and prevent damages to the sheets.
However, in cases when a prescribed process such as the stapling process is executed by accumulating a prescribed number of sheets on the stacker, a subsequent sheet cannot be conveyed to the stacker while the process is being conducted. Therefore, during this time, operations of the image forming apparatus have to be temporarily stopped, which decreases a production ratio of the image forming apparatus.
In order not to decrease the production ratio, the image forming apparatus is required of continued operation even while the prescribed process is executed by accumulating the sheets on the stacker.
For this reason, it is required to convey the sheets carried in the conveyance unit for the stacker (conveyance unit the entry of stacker) toward another conveying destination, and to make them wait until becoming possible to be accumulated in the stacker. In this another conveying destination, a conveyance unit (conveyance roller) is required of returning the sheets having waited to the conveyance unit at the entry of the stacker.
In cases of conveying the sheet carried into the conveyance unit at the stacker entry toward a waiting conveyance path, if stiffness is imparted as the case of ejecting the sheet to the stacker, being affected by wave shaped corrugation in width direction of the sheet caused by imparting the stiffness, paper jamming may occur at conveyance rollers for returning sheet. More specifically, if the stiffness imparting is controlled only by the sheet conditions, as described in Patent document 1, there may be cases of causing the paper jam at the conveyance rollers for returning sheet.
The present invention is accomplished in view of the above situations, and an objective of which is to provide a post-processing apparatus that, in cases of executing post-processing by accumulating sheets in the stacker, does not cause a failure accumulation of sheet in the stacker or damage of sheet, does not cause a sheet jam at the time of sheet conveyance, and further does not cause decrease of production ratio of the image forming apparatus.